1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display and a driving method thereof.
2. Description of the Related Art
An organic electroluminescent display electrically excites, for example, a phosphorous organic compound to emit light, and drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell may include an anode (for example, made of ITO), an organic thin film, and a cathode layer (for example, made of metal). The organic thin film may have a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and for improving emitting efficiencies. It may also include an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emitting cells may include a passive matrix method, and an active matrix method using thin film transistors (TFTs) or MOSFETs. The passive matrix method includes cathodes and anodes that cross each other, and drives by selecting lines. The active matrix method couples a TFT and a capacitor to each indium tin oxide (ITO) pixel electrode to thereby maintain the voltage by capacitance.
FIG. 2 shows a pixel circuit for driving an organic electroluminescent element of an organic electroluminescent device using TFTs for one of the N×M pixels.
As shown in FIG. 2, a current driven transistor Mb is coupled to the organic electroluminescent element of the organic electroluminescent device, and supplies a current for light emission. The amount of current of the current driven transistor Mb is controlled by a data voltage applied through a switching transistor Ma. In this instance, a capacitor C for maintaining the applied voltage may be coupled between a source and a gate of the transistor Mb. A gate of the transistor Ma may be coupled to the nth scan line Scan[n], and a source thereof may be coupled to the data line Data[m].
As to the operation of the above-described pixel circuit, when the transistor Ma is turned on (because of a scan signal applied to the gate of the switching transistor Ma) a data voltage VDATA is applied to the gate of the driving transistor Mb through the data line. Accordingly, a current correspondingly flows to the organic electroluminescent element through the transistor Mb to allow the organic electroluminescent element to emit light.
The current flowing to the organic electroluminescent element may be given by Equation 1:
                                          I            OELD                    =                                                    β                2                            ⁢                                                (                                                            V                      GS                                        -                                          V                      TH                                                        )                                2                                      =                                          β                2                            ⁢                                                (                                                            V                      DD                                        -                                          V                      DATA                                        -                                          V                      TH                                                        )                                2                                                    ,                            Equation        ⁢                                  ⁢        1            in which IOELD is a current flowing to the organic electroluminescent element, VGS is a voltage between the source and the gate of the transistor Mb, VTH is a threshold voltage of the transistor Mb, VDD is a power supply voltage, VDATA is a data voltage, and β is a constant. β may be found according to Equation 2.
                              β          =                      μ            ⁢                                                  ⁢                          Cox              (                              W                L                            )                                      ,                            Equation        ⁢                                  ⁢        2            
in which μ is the mobility of electrons or holes, Cox is capacitance of an oxide film, W is a channel width, and L is a channel length.
As given in Equation 1, the current corresponding to the applied data voltage VDATA may be supplied to the organic electroluminescent element of the organic electroluminescent device, and the organic electroluminescent element may emit light in response to the supplied current in the pixel circuit shown in FIG. 2. In such a case, the applied data voltage VDATA may have multi-level values within a predetermined range in order to represent gray scales.
However, the conventional electroluminescent display increases power consumption and brightness in high temperature operation because higher temperature leads to higher mobility of the electrons or holes, thereby increasing the value of β. Resultantly, the current flowing to the organic electroluminescent element may increase as can be seen by examination of Equation 2. More current can increase resistance-induced heat, which (in turn) can lead to yet higher current levels. This unwanted feedback mechanism is known as thermal runaway.